Optimum CG Range

Bruce wrote:
Eric Greenwell wrote:

Why is the ability to stall in a steep turn a useful criteria? It sounds
like a safety problem to me.

I expect it has to do with efficiency.

If your CG is such that your control inputs are minimised - you reduce
drag. In the case of steep thermalling, it reduces safety - because
you can now stall and theoretically spin.

There can be few things as frustrating as my experience with my (new
to me) Kestrel 19. First flight I wanted to be cautious so set the CG
at 35%. Then the day was booming - but with tight strong thermals, and
I was continually running out of elevator. Stick against the back stop
and the thermal is still tighter.

My Cirrus with it's all flying tail never had that problem. Of course
you could depart controlled flight if you got too enthusiastic...

At 35%, you are a long way from stalling in a steep turn. With a more
rearward CG, you would turn tightly enough, but still without enough
elevator to stall in a "tight" turn.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

On Dec 14, 7:55*pm, Eric Greenwell wrote:
Herb wrote:
Another useful approach is start at about 66% aft using manufacturer's
CG range. When making the tightest turn you normally do, if you run
out of elevator, you need to shift CG back a bit. You will probably
end up around 75%.There isn't a huge benefit in having the CG way
back, but there is a significant deterioration of handling which
requires better pilot skills to offset.The last little bit of glider
performance costs quite a bit in pilot workload until you are very
proficient. I usually take a couple pounds out of the tail in the
Spring and put it back in when my skills are back up to snuff.
FWIW
UH

Here's another gem piece of advice: With my 3D model airplanes I roll
inverted and check if I need down elevator to stay level. *If so, the
cg needs to be moved further back. *A well set up model will happily
fly inverted without elevator movement! *Haven't tried that in my LS8,
though.
Seriously, *Hanks and Eric's methods will both work well. *As long as
inside the book range, find the cg that gives you good handling and
enough up elevator to stall the plane in a steep turn. *It'll be at
75%-90% aft.

Why is the ability to stall in a steep turn a useful criteria? It sounds
like a safety problem to me.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly- Hide quoted text -

- Show quoted text -

Gives adequate pitch authority to pull to max lift coefficient, thus
tightest turn. From my experience, this is usually about 75-80% aft in
manufacturer's approved range.
UH

Thank you UH, P3 and J7 for your thoughts on optimal CG location. My
current CG on the LS8 lies between 45% to 55% of the available range,
depending on how well my diet is working. :-( I’ll start next
Spring without any rear weight until my proficiency returns, then move
the CG back to 65%, then 70%, etc. I’ll quit at 80%, which for me is 1
gal of water in the tail tank and thus serves as a very clear stopping
point.

-John

On Dec 15, 6:52*am, wrote:

Gives adequate pitch authority to pull to max lift coefficient, thus
tightest turn. From my experience, this is usually about 75-80% aft in
manufacturer's approved range.
UH


It's not quite that simple though is it?

For the ASW-28, and probably other modern gliders, the "manufacturer's
approved CG range" is dependent on the glider mass. Again for the
28, a cg position of 75-80 of approved range at min weight (315-321 mm
aft of root leading edge) will be behind the approved aft CG limit at
max gross wt (306mm).

I used to think that the change in aft cg limit with increasing mass
was to protect for the case where the tail tank fails to dump. If
that is true then ASW 28 built without the optional tail tank would
not have the variable aft limit. Do they?

Comments or other explanations of the variable aft limit?

Hank - Where is your 28 CG at max gross or at the max weight you fly
at if lower?

Andy (GY)

wrote:

Why is the ability to stall in a steep turn a useful criteria? It sounds
like a safety problem to me.
Gives adequate pitch authority to pull to max lift coefficient, thus
tightest turn. From my experience, this is usually about 75-80% aft in
manufacturer's approved range.
UH

How steep is the turn for which you still desire the ability to stall?
And how close to the ground or hillside do you expect to need this ability?

It's not clear to me this would be a useful ability for a pilot that is
trying to thermal away from a hillside, mountain, or during a low save.

--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly

Herb wrote:
...find the cg that gives you good handling and
enough up elevator to stall the plane in a steep turn. It'll be at
75%-90% aft.

Herb, J7

Reminds me of the old rule of thumb, if you could call it that:
CG too far forward, can't pull the nose up for takeoff.
CG too far aft, can't recover from a stall/spin.

This method can be expensive in airplanes though... :-)

Brian W